Nacreous pigments exhibit pearl-like and/or iridescent effects upon the transmission and reflection of light therethrough. As is well known in the art, the characteristics of such pigments depends upon optical interference phenomena as more fully described, for example, in "The Properties of Nacreous Pigments," Greenstein and Miller, Technical Papers, Vol. XIII, Annual Technical Conference, Society of Plastic Engineers, May 1967.
Nacreous pigments are conventionally formulated for use in suspensions of light transmitting resinous media which can be applied by dipping or spraying operations to form plastic coatings or by extruding, molding, casting, or like techniques to provide solid plastic articles incorporating such pigments. Nacreous pigments so utilized should have indexes of refraction which differ from the suspending media because the pearly or nacreous effect displayed by such pigments depends upon the difference between the index of refraction of the oriented, plate-like pigment particles and the index of refraction of the medium in which they are dispersed.
Mica by itself is not a satisfactory nacreous pigment inasmuch as its average index of refraction is about 1.58 which is too close to the index of conventional transparent resinous media of about 1.50-1.59. Excellent nacreous pigments may, however, be provided by the deposition of titanium dioxide or iron oxide coatings on mica flakes.
Linton U.S. Pat. No. 3,087,828 describes the preparation of titanium dioxide coated mica nacreous pigments which optionally can be topped with a layer of iron oxide, the latter layer amounting to 0.5 to 20 weight percent of the titanium dioxide. The patent indicates that the titanium dioxide layer is at least 10 weight percent of the pigment and indicates that 10 weight percent is equivalent to about 50 mg. TiO.sub.2 /m.sup.2 mica or TiO.sub.2 geometric thickness of about 30 mu.
Linton U.S. Pat. No. 3,087,829 describes a pigment made by depositing hydrated iron oxide on mica and the coating is then dehydrated and crystallized by means of heat resulting in an iron oxide coated mica pigment. The iron oxide constitutes 10 to 40 weight percent of the pigment.
Brand et al U.S. Pat. No. 3,711,308 teaches a coated mica pigment in which a first coat contains titanium or zirconium oxide mixed with iron oxide and a second layer of only the titanium or zirconium oxides which is about twice as thick as the first coating. Bernhard et al. U.S. Pat. No. 3,874,890 teaches a TiO.sub.2 coated mica pigment which is top coated with iron oxide in an amount up to 20 percent of the final pigment. The TiO.sub.2 coating is about 100-300 mg. TiO.sub.2 /m.sup.2 of mica surface which, based on Linton's teachings, means that the geometric thickness of the titanium layer is about 60 mu.
Bernhard et al U.S. Pat. No. 3,926,659 teaches that in a mica/TiO.sub.2 or ZrO.sub.2 /Fe.sub.2 O.sub.3 pigment, the TiO.sub.2 or ZrO.sub.2 interlayer can be reduced to as little as 5 mg/m.sup.2 if the iron is initially deposited as a single form of iron (III) oxide hydroxide, viz., only one of .alpha.,.beta. or .gamma.-FeOOH or magnetite. Aluminum oxide can be coated on top of an iron oxide coated mica as a protective layer.
Coatings which are entirely Fe.sub.2 O.sub.3 are desirable in order to obtain dark metallic looking pigments such as "bronze" or "copper." A serious defect in pigments containing only Fe.sub.2 O.sub.3 coatings, however, is that the Fe.sub.2 O.sub.3 layer suffers from poor adhesion to the mica substrate.
Any section of the iron oxide layer which is dislodged or stripped from the mica tends to break up into small particles which scatter light in all directions thereby decreasing the directional, specular character required for nacreous luster. Additionally, no reflection takes place at the exposed mica surface because the refractive index of mica (about 1.58) is close to refractive index of most plastic media (about 1.50-1.59) in which the nacreous pigments are employed. As a result, the nacreous quality or pearly luster of the pigment is diminished.
When nacreous pigments are incorporated into plastics and the plastics are further processed, the plastic composition is subjected to relatively high shear forces. Under such conditions, a minor proportion of the Fe.sub.2 O.sub.3 surface film is stripped from the mica plate. Consequently, the nacreous luster of the finished plastic articles is muted.
It is known experimentally that better adhesion of TiO.sub.2 to the mica substrate can be obtained if higher calcining temperatures are used. In the case of Fe.sub.2 O.sub.3, however, higher calcining temperatures do not improve the adhesion but only result in a decrease in luster.
It has now been discovered that dark colors and very good adhesion of the Fe.sub.2 O.sub.3 layer can be obtained if a very thin layer of TiO.sub.2 or Al.sub.2 O.sub.3 is deposited on the mica below the iron oxide layer, and it is not necessary that the iron be deposited initially in a single form. The pigments withstand high shear forces without separation of the iron oxide layer. The pigment platelets therefore retain their integrity and the desired luster and metallic color are exhibited in plastic objects. At the same time, the colorless titanium or aluminum oxide does not have a significant effect on the dark color because its concentration is too low.
TiO.sub.2 layers on mica are usually deposited on mica from either a titanium tetrachloride or titanyl sulfate coating bath. Adhesion of the TiO.sub.2 is good when TiCl.sub.4 is employed. For ease of operation and simplicity, however, it is preferred to use titanyl sulfate. Unfortunately, titanyl sulfate produced TiO.sub.2 coated mica is subject to considerable stripping. It was therefore most surprising to find that very thin titanyl sulfate produced TiO.sub.2 interlayered Fe.sub.2 O.sub.3 coated mica exhibited good resistance to stripping.
It is the object of this invention to provide new and improved iron oxide coated mica nacreous pigments which are characterized by being dark metallic looking such as bronze or copper and by a very good adhesion of the Fe.sub.2 O.sub.3 layer. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.